1. Field of the Invention
This invention relates to the field of clutch assists for a motor vehicle. More particularly, the invention pertains to a booster system parallel to a conventional manual or hydraulic clutch actuation system.
2. Description of the Prior Art
A manual transmission for use in an automotive vehicle is generally connected to the engine through a friction clutch whose engagement and disengagement is determined by the manual movement of a clutch pedal typically controlled by the vehicle operator with his foot. The efforts in many such applications to release the clutch pedal may be greater than desired for some drivers, particularly in heavy trucks.
A system to reduce the effort required to release a clutch pedal is described in U.S. Pat. No. 4,871,051 ('051) to Whitmer. The system in the '051 patent uses a control valve connected between the clutch pedal and clutch operating lever, so when the clutch pedal is depressed, the linkage rod is compressed to displace a spool in a control valve. As the spool is displaced, the control valve delivers compressed air to a booster cylinder. The booster cylinder in turn applies a proportional force to the clutch operating lever. The '051 patent states in column 3, lines 19-23 that when the air supply to the booster cylinder is lost, the spool in the control valve is pushed against the opposite end of the valve body, and the linkage is pushed directly through the valve.
The '051 patent does not easily package in many designs, because the valve must be mounted on the linkage. Additionally, the movement of the valve requires additional travel of the pedal before the clutch is disengaged. For example, the air supply is exhausted when the pedal is not pressed. Thus the initial force would compress the spring and apply compression to the linkage proportional to the spring rate. The deflection of the spring requires additional travel of the clutch pedal. Upon loss of air pressure in the '051 patent, the valve must be fully stroked before the operator's full force is applied to the linkage, resulting in further pedal travel and slower response in the clutch system. Finally, the proportional force applied by the assist cylinder in the '051 patent results in a clutch system which does not provide the normal clutch pedal feel to the operator.
A second clutch assist design is shown in U.S. Pat. No. 5,042,631 ('631) to Ellenberger et al. The '631 design uses a pneumatic cylinder to cause movement of the clutch. The cylinder receives pressure from a valve which receives input from a sensor based on displacement of the clutch pedal. The valve provides a predetermined air pressure to a pneumatic cylinder to move the clutch based on the displacement of the clutch pedal.
The system shown in the '631 patent relies on air pressure to actuate the cylinder, which in turn operates the clutch; therefore there is an inherent delay in the application of force to the pedal until the valve moves and supplies air to the cylinder, plus the clutch feel to the operator does not come from feedback from the clutch. Additionally, there is no provision in the '631 patent to actuate the clutch if pressure is lost to the cylinder. The feedback (forces) the operator would feel would be that to energize the sensor, not feedback from the clutch. Finally, this system is not easily adaptable to an existing vehicle, as the system shown in the '631 patent would completely replace the conventional system.
It would be desirable to provide a clutch assist mechanism which is adaptable to be retrofitted into existing vehicles, or installed as an original equipment option on new vehicles. Furthermore, such a mechanism should provide the operator with a feel for the clutching forces, be simple, relatively inexpensive, and provide for clutching even when the assist power is lost.